Question About Selling Stock Photos

I'm interested in uploading some photos to sell on stock image sites like Bigstock, Getty, and Image Shack. I've heard that you should submit photos to more than one website to insure they'll be noticed by the widest audience, and in case your photos are rejected by your first choice of website. My question is if I submit a photo to one website and it's accepted, are there any special legal considerations I should be aware of if I want to submit that same photo to other stock image sites?

I ask because I've seen some photos available on more than one stock image site and this is something I've never done before. My goal is to amass a sizable online photo library over time in the hopes of making a modest profit. If anyone can offer suggestions or helpful info in this area, it's greatly appreciated.

Your post has nothing to do with graphic design and is actually about sales. We moved it here to our off topic forum.

I might suggest you look at a site called 500px.com, which is an excellent showcase for professional photographers and has a built-in feature that helps sell your work to stock agencies.